College of LAS « Illinois

Geology

Lost and Found

It wasn’t exactly a job for your average lost and found department. What had gone missing was a large chunk of the Earth; and it had been missing for roughly 15 million years.

Using seismic waves, LAS geologists recently found this massive, missing piece of the Earth’s lithosphere—the outer, rocky part of the planet. What’s more, their discovery helps to confirm a theory that this piece of the planet had sunk into the mantle deep within the Earth millions of years ago.

Until recently, this tantalizing theory lacked any clear observation. But now, a team led by Wang-Ping Chen, University of Illinois professor of geophysics, has discovered the block of errant rock beneath Tibet.

The Tibetan Plateau and the adjacent Himalayan Mountains were created by the movements of vast tectonic plates that make up the Earth’s outermost layer of rocks. About 55 million years ago, the Indian plate crashed into the Eurasian Plate, forcing the land to slowly buckle and rise.

Many scientific models have projected what might have happened when this massive collision occurred. One theory said the Eurasian lithosphere became thicker as the two plates butted against each other. Then the thickened lithosphere became unstable, causing a piece to break off and sink into the deep mantle within the Earth.

To test this theory through a project called Hi-CLIMB, Chen and his doctoral student, Tai-Lin Tseng, measured the velocity of seismic waves traveling beneath the Tibet region at depths of 300 to 700 kilometers (186 to 435 miles). Seismic waves travel faster through colder rock, such as the missing chunk of lithosphere. So, by analyzing the seismic waves, Tseng was able to locate the position of the missing rock in the midst of the hotter material that makes up the mantle.

“We not only found the missing piece of cold lithosphere, but we also were able to reconstruct the positions of tectonic plates back to 15 million years ago,” Tseng says. “It therefore seems much more likely that instability in the thickening lithosphere was partially responsible for forming the Tibetan Plateau.”

Averaging 16,000 feet in elevation, the Tibetan Plateau is the world’s largest and highest plateau.

In addition to shedding light on an important theory, Tseng says this discovery “is fundamental” in understanding the full dynamics of collisions between tectonic plates.